Synchronous self-shifting clutches

ABSTRACT

In pawl and ratchet mechanism capable of being set selectively to a pawl free condition or to a ratcheting condition, baulking means are provided which operate in response to relative rotation of the pawls and ratchet teeth to permit or prevent a change from the pawl free condition to the ratcheting condition according to whether the direction of relative rotation is appropriate or inappropriate for the change to be made. The invention is applicable to toothed clutches wherein pawl and ratchet mechanism is employed for automatically aligning the clutch teeth for interengagement.

United States Patent Heybourne et al.

Feb. 6, 1973 SYNCHRONOUS SELF-SHIFTING CLUTCHES inventors:

Assignee:

Filed:

Appl. No.:

Robert Howard Heybourne, East Molesey, Surrey; Herbert Arthur Clements,Oatlands Park, Weybridge, Surrey, both of England S.S.S. PatentsLimited, London, England May 3, 1971 Foreign Application Priority DataMarch l l, 1970 Great Britain ..l 1,614/70 US. Cl.

Int. Cl

....-.....l92/67 A, 192/53 G, 192/53 H,

....................................... ..Fl6d 23/08 Field ofSearch....l92/46, 47, 53 G, 53 H, 67 A,

l92/ll4R [56] References Cited UNITED STATES PATENTS 2,908,367 lO/l959Sinclair ..l92/53 H X 2,913,084 11/1959 Short ..i ......l92/67 A PrimaryExaminer -Allan D. Hermann Attorney-Woodcock, Phelan & Washburn [57]ABSTRACT In pawl and ratchet mechanism capable of being set selectivelyto a pawl free condition or to a ratcheting condition, baulking meansare provided which operate in response to relative rotation of the pawlsand ratchet teeth to permit or prevent a change from the pawl freecondition to the ratcheting condition according to whether the directionof relative rotation is appropriate or inappropriate for the change tobe made.

The invention is applicable to toothed clutches wherein pawl and ratchetmechanism is employed for automatically aligning the clutch teeth forinterengagement.

10 Claims, 10 Drawing Figures PATENTEDFEB 6 ms 3.715.019

v SHEET 1 or 3 18 Fla. 7.

SYNCHRONOUS SELF-SHIFTING CLUTCHES This invention relates to pawl andratchet mechanism and particularly but not exclusively to pawl andratchet mechanism incorporated in a toothed clutch for automaticallyaligning the clutch teeth for interengagement upon relative rotation ofthe clutch input and output parts in one direction. Such mechanisms arenormally unidirectionally free, that is to say for one direction ofrelative rotation of the elements of the pawl and ratchet mechanism,viz. the pawls on the one hand and the ratchet teeth on the other hand,the mechanism overruns with the pawls ratcheting relative to the ratchetteeth, whereas for the other direction of relative rotation pawls engageratchet teeth.

In some applications of such mechanisms it is necessary to makeprovision for a bi-directionally free condition. Such a condition mayfor example be achieved by shifting the pawls axially away from theratchet teeth, or conversely by shifting the ratchet teeth axially awayfrom the pawls, to a position in which they are incapable of ratcheting,or by turning the pawls to positions in which their noses are out of thepaths of the ratchet teeth. Such a condition may be termed a pawl freecondition.

For the mechanism to be returned from the pawl free condition to aunidirectionally free condition in which the pawls are in ratchetingrelationship with the ratchet teeth it is essential that the directionof relative rotation of the elements of the pawl and ratchet mechanismshould be in the overrunning direction; since the effect of attemptingto return the mechanism from the pawl free condition to the ratchetingcondition with the said elements in rapid relative rotation in thedirection opposite to the direction for overrunning would be that pawlsand rachet teeth would come into violent contact resulting in damage tothe pawls and/or to the ratchet teeth.

The object of the invention is to overcome this problem, and inaccordance with the invention means are provided which are responsive torelative rotation of the elements of the pawl and ratchet mechanism toprevent a change from the pawl free condition to the unidirectionallyfree ratcheting condition when the direction of relative rotation of thesaid elements is inappropriate for such a change, but to permit a changefrom the pawl free condition to the unidirectionally free ratchetingcondition when the direction of relative rotation of said elements isappropriate for such a change.

In the accompanying drawings,

FIG. 1 is a view in side sectional elevation of a synchronousself-shifting toothed clutch embodying the invention. The upper half ofthe Figure shows the clutch in a pawl free viz. bi-directionally freecondition and the lower half of the Figure shows the clutch in aunidirectionally free condition in which the pawls are in ratchetingrelationship with the ratchet teeth,

FIG. 2 is a view of the upper half of the clutch in side sectionalelevation, showing the clutch in the engaged condition,

FIG. 3 is a sectional view on the line III-III of FIG. 1

FIGS. 4, and 6 are detail sectional views, on a larger scale than FIG.1, showing baulking teeth and a baulking ring in the relative positionsthat they assume for various conditions of the clutch,

FIGS. 4a, 5a and 6a are development views of baulking teeth and blockingteeth, the Figures corresponding respectively to FIGS. 4, 5 and 6, and

FIG. 7 is a detail view illustrating a modification of the baulkingteeth and blocking teeth.

Referring to the drawings, the clutch includes a first part 1 formedwith external left-hand helical splines 2 with which are engagedinternal helical splines 3 in an intermediate member 4, which is therebyconstrained for helical movement relative to the clutch part 1, the saidhelical movement being limited in one direction by an axial stop formedby an annular shoulder 5 on the.

clutch part 1 and in the other. direction by an axial stop formed by anannular flange 6 on a ring 7 fixed to the part 1, the flange 6cooperating with a radially inwardly projecting annular flange 7 in theintermediate member 4. The intermediate member 4 is formed with a ringof external clutch teeth 8, and carries pawls 9 the noses of which pointin anticlockwise direction as viewed from the righthand end of theclutch (FIG. 3). The pawls 9 are mounted on pawl pins 10 and areprovided with control springs 11 which urge the noses of the pawlsradially outwardly. The intermediate member 4 is also formed with a ringof external baulking teeth 12, each of which (FIGS. 40, 5a and 6a) has astraight flank l3 and a flank 14 which is stepped to form a part 15 ofreduced circumferential thickness at one end of the tooth. As seen fromthe left-hand. end of the clutch in FIG. 1, the straight flanks 13 ofthe baulking teeth 12 face in the anticlockwise direction and thestepped flanks l3 face in the clockwise direction.

A second clutch part is constituted by a sleeve 16 carrying a ring ofinternal clutch teeth 17 and a ring of internal ratchet teeth 18. Thesleeve 16 also carries annular members 19 and 20 shaped so as to providebetween them an internal groove, in which is accommodated a ring 21forming part of a blocking tooth carrier. A white metal bearing 21' isprovided between the ring 21 and its groove. The carrier also includes acylindrical member 22 which projects from the inner periphery of thering 21 and carries a ring of internal blocking teeth 23 which areinterengaged with the baulking teeth 12.

With the clutch in the unidirectionally free condition, as shown in thelower half of FIG. 1, the external clutch teeth 8 are out of engagementwith the internal clutch teeth 17, and the pa'wls 9 are in ratchetingrelationship with the ratchet teeth 18. With the clutch output part 16rotating in anticlockwise direction relative to the clutch input part 1as seen from the right-hand end of the clutch (see FIG. 3), the pawls 9ratchet relative to the ratchet teeth 18 and the clutch overruns.

The ring 21 forming part of the blocking tooth carrier is rotatablyslidable in its lubricated bearing 21 When the clutch parts 1 and 16 arein relative rotation in the overrunning direction as :above described,the drag on the ring 21 of the blocking tooth carrier, due to thefriction between the ring 21 and the bearing 21', is such that theblocking teeth 23 are maintained in com tact with the straight flanks 13of the baulking teeth 12.

When the direction of relative rotation ofthe clutch parts 1 and 16reverses, pawls 9 engage ratchet teeth 18 and the intermediate member 4is thereby shifted helically, to the right in FIG. 1, along the inputpart 1 so as to bring the external clutch teeth 8 precisely into initialinterengagement with the internal clutch teeth 17,

whereupon the interaction of the clutch teeth 8 and 17 in conjunctionwith the helical splines 2 and 3 draws the intermediate member 4 intofull toothed engagement with the second clutch part 16 and against theaxial stop 5, so that the clutch is engaged (FIG. 2). When the directionof relative rotation of the clutch parts 1 and 16 again reverses, theinteraction of the clutch teeth 8 and 17 in conjunction with the helicalsplines 2 and 3 shifts the intermediate member 4 to the left so as tobring the external clutch teeth 8 out of engagement with the internalclutch teeth 17 and to bring the pawls 9 back into ratcheting engagementwith the ratchet teeth 18, as shown in the lower half of FIG. 1. Duringthese movements of the intermediate member 4 the straight flanks 13 ofthe baulking teeth 12 slide relative to the blocking teeth 23.

As so far described, the clutch is unidirectionally free, that is to sayfor one direction of relative rotation of the clutch parts 1 and 16 theclutch overruns with the pawls 9 ratcheting relative to the ratchetteeth 18, whereas upon reversal of the direction of relative rotation ofthe clutch parts 1 and 16 the clutch engages as above described.

In order to bring the clutch to a condition of bidirectional freedom,viz., the pawl free condition, the intermediate member 4 is shifted tothe left from the position shown in the lower half of FIG. 1 to aposition in which the pawls 9 are axially clear of the ratchet teeth 18,as shown in the upper half of FIG. 1. This movement of the intermediatemember 4 may for example be effected by operating an axially movablecontrol fork 24 engaged with lost motion in an external annular groove25 in the intermediate member 4. During this movement of theintermediate member 4 the baulking teeth 12 move so far to the left thatthe blocking teeth 23 move on to the parts 15 of the baulking teeth 12of reduced circumferential width. So long as the direction of relativerotation of the clutch part 1 and intermediate member 4 relative to theclutch part 16 corresponds to the ratcheting direction of relativerotation of the pawls 9 and ratchet teeth 18, as shown by the arrow inFIG. 4, the blocking teeth 23 are held in contact with the straightflanks 13 of the baulking teeth 12 (FIG. 4a) by the frictional drag onthe blocking tooth carrier due to the friction between the ring 21 andthe surfaces of the bearing 21' in which it is accommodated. Hence it ispossible, by operating the control fork 24, to shift the intermediatemember 4 to the right to bring the pawls 9 into ratcheting relationshipwith the ratchet teeth 18 (lower half of FIG. 1). If however while inthe pawl free condition, the direction of relative rotation of theclutch parts 1 and 16 is in the opposite direction (indicated by thearrow in FIG. the blocking teeth 23 are held, by frictional drag,against the flanks of the narrow parts of the baulking teeth 12, on thestepped sides 14 thereof as shown in FIG. 5a. Hence if an attempt ismade to shift the intermediate member 4 to the right the steps on thebaulking teeth 12 come into axial engagement with the blocking teeth 23so as to prevent further movement of the inter mediate member 4 to theright such as would bring the pawls 9 into contact with the ratchetteeth 18 and cause damage.

The operator is therefore obliged to wait for the direction of relativerotation of the clutch parts 1 and 16 to reverse before the intermediatemember 4 can be shifted to the right. When such reversal takes place,the frictional drag on the blocking tooth carrier 21, 22 brings theblocking teeth 23 on to the straight flanks of the baulking teeth 12(FIG. 4a), so that there is no longer any obstruction to movement of theintermediate member 4 to the right to bring the pawls 9 into ratchetingrelationship with the ratchet teeth 18.

If, with the clutch in the pawl free condition (FIG. 6) the intermediatemember 4 is shifted to the right whilst the clutch parts 1 and 16 arestationary relatively to one another, intermediate member 4 moveshelically relative to the clutch output part 16 and the baulking teeth12 move helically relative to the blocking teeth 23, as shown by thearrow in FIG. 6a. When the clutch is in the pawl free condition theblocking teeth 23 are spaced sufficiently far in the axial directionfrom the steps of the baulking teeth 12 such that upon movement of theintermediate member 4 to the right the helical movement of the baulkingteeth 12 relative to the blocking teeth 23 causes the steps of thebaulking teeth 12 to pass clear of the blocking teeth 23, so that themovement of the intermediate member 4 to the pawl ratcheting position isunimpeded. Hence the intermediate member 4 can be shifted from the pawlfree condition to the ratcheting condition when the clutch parts 1 and16 are stationary relatively to each other without being impeded by thebaulking means.

The frictional drag exerted on the rings 21 of the blocking toothcarrier may be arranged to be higher for one direction of relativerotation of the ring 21 in its bearing 21 than for the other direction,such as to assist disengagement of the blocking teeth 23 from the stepsof the baulking teeth 12 when the direction of relative rotation of theclutch parts 1 and 16 become appropriate for shifting of theintermediate member 4 from the pawl free position to the pawl ratchetingposition.

As illustrated in FIG. 7, the baulking teeth 12 and the blocking teeth23 may be undercut so as to provide an interlocking action between thebaulking teeth and the blocking teeth.

In a modification (not illustrated) instead of a ring of baulking teethand a ring of blocking teeth, there may be provided a single baulkingtooth and a single blocking tooth. In this case, in the pawl freecondition the blocking tooth will bear against the straight flank of thebaulking tooth when the direction of relative rotation of the elementsof the pawl and ratchet mechanism is appropriate for a change to theunidirectionally free condition, whereas the blocking tooth will bearagainst the narrow part of the stepped flank of the baulking tooth whenthe direction of relative rotation of the said elements is inappropriatefor such a change.

The invention may be incorporated in synchronous self-shifting clutcheswherein an intermediate member is movable axially relative to the secondclutch member and the movement of the intermediate member to initiateinterengagement of the clutch teeth is brought about by the coaction ofpawls and ratchet teeth having helical coacting faces, as described inUnited States Pat. No. 3,458,021.

What is claimed is:

l. Pawl and rachet mechanism of the type comprising a first rotaryelement, at least one pawl carried by said first rotary element, asecond rotary element, at least one ratchet tooth carried by said secondrotary element, said mechanism further being of the type that includesmeans mounting said first and second rotary elements for relativemovement to and from a pawl free condition of the mechanism,characterized by impeding means which, with the mechanism in the pawlfree condition, are responsive to relative rotation of said first andsecond rotary elements in a direction which is inappropriate for achange of the mechanism from the pawl free condition to assume anoperative condition in which said impeding means impede such a change,said impeding means being responsive to relative rotation of said firstand second rotary elements in the other direction to assume aninoperative condition in which a change of the mechanism from the pawlfree condition is permitted.

2. Pawl and ratchet mechanism according to claim 1 wherein the saidelements are mounted for relative axial movement from a ratchetingcondition to a pawl free condition and vice versa, and wherein the saidimpeding means comprise blocking teeth rotatable with one of saidelements, and baulking teeth movable angularly relative to the other ofsaid elements, the baulking teeth each having a flank against which aflank of a blocking tooth bears when said elements are in a pawl freecondition and are in relative rotation in the direction appropriate fora change from the pawl free condition, said flanks being mutually shapedto permit said change, and said baulking teeth each having a flankagainst which a flank of a blocking tooth bears when said elements arein a pawl free condition and are in relative rotation in the directioninappropriate for such a change, said last-mentioned flanks of thebaulking teeth and blocking teeth being mutually shaped to provideobstruction to such a change.

3. Pawl and ratchet mechanism according to claim 1 wherein the saidelements are mounted for relative axial movement from a ratchetingcondition to a pawl free condition and vice versa, and wherein the saidimpeding means comprise blocking teeth rotatable with one of saidelements and baulking teeth interengaged with said blocking teeth andangularly movable relative to the other of said elements, the baulkingteeth each having a flank against which, in the pawl free condition ofthe mechanism, a blocking tooth bears when the direction of relativerotation of said elements is inappropriate for a change from the pawlfree condition, said flank being stepped to obstruct relative movementof the baulking tooth and blocking tooth corresponding to such a change,said baulking teeth each having a further flank against which, in thepawl free condition of the mechanism, a blocking tooth bears when thedirection of relative rotation of said elements is appropriate for achange to the pawl free condition, which further flank is shaped topermit such change without substantial obstruction.

4. A toothed clutch comprising input and output parts, engageable anddisengageable sets of clutch teeth, and paw] and ratchet mechanismaccording to claim 3 for automatically aligning said sets of clutchteeth for interengagement upon relative rotation of said input andoutput parts in one direction.

5. A synchronous self-shifting toothed clutch comprising a first rotaryclutch part, clutch teeth carried by said first clutch part, a secondrotary clutch part, an intermediate member, clutch teeth on saidintermediate member, means constraining said intermediate member forhelical movement relative to said second clutch part to bring the clutchteeth of said intermediate member into and out of interengagement withthe clutch teeth of said first clutch part, and pawl and ratchetmechanism in accordance with claim 3, the said elements of said pawl andratchet mechanism being carried by said first clutch member and saidintermediate member respectively, said pawl and ratchet mechanism, whenout of the pawl free condition, being effective upon passage of saidfirst and second clutch parts through synchronism in one direction ofrelativerotation, to shift said intermediate member into at leastinitial interengagement of its clutch teeth with the clutch teeth ofsaid first clutch member.

.6. Pawl and ratchet mechanism according to claim 1 wherein the saidimpeding means include at least one baulking member rotatable with oneof said elements and at least one blocking member angularly movablerelative to the other of said elements, said baulking member and saidblocking member being mutually shaped such that a change from the pawlfree condition is impeded by the interaction of said blocking member andsaid baulking member when the direction of relative rotation of saidelements is inappropriate for such a change, whereas such change isunimpeded when the direction of relative rotation of said elements is appropriate for such a change.

7. Pawl and ratchet mechanism according toclaim 1 wherein the said meansinclude a baulking tooth rotatable with one of said elements and ablocking toothangularly movable relative to the other of said elements,said baulking tooth having a flank against which, in the pawl freecondition of the mechanism, a flank of said blocking tooth bears whenthe direction of relative rotation of said elements is appropriate for achange from the pawl free of the mechanism, which flanks are mutuallyshaped to permit relative shifting of said baulking tooth and saidblocking tooth corresponding to such a change, said baulking toothhaving a further flank against which a further flank of said blockingtooth bears when, in the pawl free condition of the mechanism, thedirection of relative rotation of said elements is inappropriate forsuch a change, said further flanks being mutually shaped to provideobstruction to selective shifting of said baulking and said blockingtooth corresponding to such a change.

8. A clutch comprising input and output parts, engageable anddisengageable sets of clutch teeth, and pawl and ratchet mechanismaccording to claim 1 for automatically aligning said sets of clutchteeth for interengagement upon relative rotation of said input andoutput parts in one direction.

9. A synchronous self-shifting toothed clutch comprising a first rotaryclutch part, clutch teeth carried by said first clutch part, a secondrotary clutch part, an intermediate member, clutch teeth carried by saidintermediate member, means constraining said intermediate member forhelical movement relative to said second clutch part to bring the clutchteeth of said intermediate member into and out of interengagement withthe clutch teeth of said first clutch part, and pawl and ratchetmechanism according to claim 1, said pawl and tive helical movementwhereby a change into and out of the pawl free condition involvesrelative helical movement of said elements, and wherein the arrangementof said impeding means is such that with said elements stationaryrelative to one another and in the pawl free condition a change from thepawl free condition can be made unimpeded by said impeding means.

1. Pawl and rachet mechanism of the type comprising a first rotaryelement, at least one pawl carried by said first rotary element, asecond rotary element, at least one ratchet tooth carried by said secondrotary element, said mechanism further being of the type that includesmeans mounting said first and second rotary elements for relativemovement to and from a pawl free condition of the mechanism,characterized by impeding means which, with the mechanism in the pawlfree condition, are responsive to relative rotation of said first andsecond rotary elements in a direction which is inappropriate for achange of the mechanism from the pawl free condition to assume anoperative condition in which said impeding means impede such a change,said impeding means being responsive to relative rotation of said firstand second rotary elements in the other direction to assume aninoperative condition in which a change of the mechanism from the pawlfree condition is permitted.
 1. Pawl and rachet mechanism of the typecomprising a first rotary element, at least one pawl carried by saidfirst rotary element, a second rotary element, at least one ratchettooth carried by said second rotary element, said mechanism furtherbeing of the type that includes means mounting said first and secondrotary elements for relative movement to and from a pawl free conditionof the mechanism, characterized by impeding means which, with themechanism in the pawl free condition, are responsive to relativerotation of said first and second rotary elements in a direction whichis inappropriate for a change of the mechanism from the pawl freecondition to assume an operative condition in which said impeding meansimpede such a change, said impeding means being responsive to relativerotation of said first and second rotary elements in the other directionto assume an inoperative condition in which a change of the mechanismfrom the pawl free condition is permitted.
 2. Pawl and ratchet mechanismaccording to claim 1 wherein the said elements are mounted for relativeaxial movement from a ratcheting condition to a pawl free condition andvice versa, and wherein the said impeding means comprise blocking teethRotatable with one of said elements, and baulking teeth movableangularly relative to the other of said elements, the baulking teetheach having a flank against which a flank of a blocking tooth bears whensaid elements are in a pawl free condition and are in relative rotationin the direction appropriate for a change from the pawl free condition,said flanks being mutually shaped to permit said change, and saidbaulking teeth each having a flank against which a flank of a blockingtooth bears when said elements are in a pawl free condition and are inrelative rotation in the direction inappropriate for such a change, saidlast-mentioned flanks of the baulking teeth and blocking teeth beingmutually shaped to provide obstruction to such a change.
 3. Pawl andratchet mechanism according to claim 1 wherein the said elements aremounted for relative axial movement from a ratcheting condition to apawl free condition and vice versa, and wherein the said impeding meanscomprise blocking teeth rotatable with one of said elements and baulkingteeth interengaged with said blocking teeth and angularly movablerelative to the other of said elements, the baulking teeth each having aflank against which, in the pawl free condition of the mechanism, ablocking tooth bears when the direction of relative rotation of saidelements is inappropriate for a change from the pawl free condition,said flank being stepped to obstruct relative movement of the baulkingtooth and blocking tooth corresponding to such a change, said baulkingteeth each having a further flank against which, in the pawl freecondition of the mechanism, a blocking tooth bears when the direction ofrelative rotation of said elements is appropriate for a change to thepawl free condition, which further flank is shaped to permit such changewithout substantial obstruction.
 4. A toothed clutch comprising inputand output parts, engageable and disengageable sets of clutch teeth, andpawl and ratchet mechanism according to claim 3 for automaticallyaligning said sets of clutch teeth for interengagement upon relativerotation of said input and output parts in one direction.
 5. Asynchronous self-shifting toothed clutch comprising a first rotaryclutch part, clutch teeth carried by said first clutch part, a secondrotary clutch part, an intermediate member, clutch teeth on saidintermediate member, means constraining said intermediate member forhelical movement relative to said second clutch part to bring the clutchteeth of said intermediate member into and out of interengagement withthe clutch teeth of said first clutch part, and pawl and ratchetmechanism in accordance with claim 3, the said elements of said pawl andratchet mechanism being carried by said first clutch member and saidintermediate member respectively, said pawl and ratchet mechanism, whenout of the pawl free condition, being effective upon passage of saidfirst and second clutch parts through synchronism in one direction ofrelative rotation, to shift said intermediate member into at leastinitial interengagement of its clutch teeth with the clutch teeth ofsaid first clutch member.
 6. Pawl and ratchet mechanism according toclaim 1 wherein the said impeding means include at least one baulkingmember rotatable with one of said elements and at least one blockingmember angularly movable relative to the other of said elements, saidbaulking member and said blocking member being mutually shaped such thata change from the pawl free condition is impeded by the interaction ofsaid blocking member and said baulking member when the direction ofrelative rotation of said elements is inappropriate for such a change,whereas such change is unimpeded when the direction of relative rotationof said elements is appropriate for such a change.
 7. Pawl and ratchetmechanism according to claim 1 wherein the said means include a baulkingtooth rotatable with one of said elements and a blocking tooth angularlymovable relative to the other of said elements, said baulking toothhaving a flank against which, in the pawl free condition of themechanism, a flank of said blocking tooth bears when the direction ofrelative rotation of said elements is appropriate for a change from thepawl free of the mechanism, which flanks are mutually shaped to permitrelative shifting of said baulking tooth and said blocking toothcorresponding to such a change, said baulking tooth having a furtherflank against which a further flank of said blocking tooth bears when,in the pawl free condition of the mechanism, the direction of relativerotation of said elements is inappropriate for such a change, saidfurther flanks being mutually shaped to provide obstruction to selectiveshifting of said baulking and said blocking tooth corresponding to sucha change.
 8. A clutch comprising input and output parts, engageable anddisengageable sets of clutch teeth, and pawl and ratchet mechanismaccording to claim 1 for automatically aligning said sets of clutchteeth for interengagement upon relative rotation of said input andoutput parts in one direction.
 9. A synchronous self-shifting toothedclutch comprising a first rotary clutch part, clutch teeth carried bysaid first clutch part, a second rotary clutch part, an intermediatemember, clutch teeth carried by said intermediate member, meansconstraining said intermediate member for helical movement relative tosaid second clutch part to bring the clutch teeth of said intermediatemember into and out of interengagement with the clutch teeth of saidfirst clutch part, and pawl and ratchet mechanism according to claim 1,said pawl and ratchet mechanism when out of pawl free condition beingeffective, upon passage of said first and second clutch parts throughsynchronism in one direction of relative rotation, to shift saidintermediate member into at least initial interengagement of its clutchteeth with the clutch teeth of said first clutch part.